Back to EveryPatent.com
United States Patent |
5,638,377
|
Quinquis
,   et al.
|
June 10, 1997
|
Method of forwarding data packets in a multi-site network, a
corresponding communications network, and a corresponding interface
module
Abstract
The header of each packet contains a virtual channel identifier. The
virtual path identifier is a packet distribution agent enabling a packet
to be distributed when the packet is travelling over a public network, and
the virtual path identifier is a packet-routing agent enabling the packet
to be routed when said packet is travelling over a private network. Each
time a packet is transferred from a private network to a public network,
routing data contained in the virtual path identifier is shifted to a
reserved portion of a virtual channel identifier that is also contained by
the packet, and it is replaced by distribution control data. Each time a
packet is transferred in the opposite direction, the distribution control
data in the packet is replaced by private network internal routing data.
Inventors:
|
Quinquis; Jean-Paul (Lannion, FR);
Fran.cedilla.ois; Joel (Lannion, FR);
Hamidi; Ridha (Lannion, FR);
Le Padellec; Philippe (Loperhet, FR)
|
Assignee:
|
Alcatel Business Systems (Paris, FR);
France Telecom (Paris, FR)
|
Appl. No.:
|
493394 |
Filed:
|
June 21, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
370/392; 370/397; 370/409 |
Intern'l Class: |
H04L 012/56; H04L 012/66 |
Field of Search: |
370/60,60.1,94.1,58.1,58.2,58.3,79,80,82,83,99,54,110.1
|
References Cited
U.S. Patent Documents
5450406 | Sep., 1995 | Esaki et al. | 370/60.
|
5453981 | Sep., 1995 | Katsube et al. | 370/79.
|
5513178 | Apr., 1996 | Tanaka | 370/60.
|
Other References
Patent Abstracts of Japan, vol. 17, No. 629 (E-1462) 19 Nov. 1993 & UP-A-05
1999228 (Oki Electric Ind Co Ltd) Aug. 6, 1993.
Patent Abstracts of Japan, vol. 17, No. 328 (E-1385), 22 Jun. 1993 &
JP-A-05 037547 (Fujitsu Ltd) Feb. 12, 1993.
Computer Communications, vol. 16, No. 10, Oct. 1993 Jordan Hill, pp.
662-666, W. Chen et al, "Efficient Multicast Source Routing Scheme".
|
Primary Examiner: Chin; Wellington
Assistant Examiner: Vu; Huy D.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
We claim:
1. A method of forwarding data organized in data packets, in a multi-site
data-interchange network for data organized in data packets, the network
being of the type including at least two mutually distant sites, each of
which is organized in private sub-networks including a plurality of
terminals that are capable of transmitting and/or receiving packets, and
that are interconnected via a distributed public network;
each of said packets comprising a header and a data field, said header
containing at least two identifiers defining two access hierarchy levels
as seen from said public network, namely a first identifier referred to as
the "virtual channel identifier" (VCI) enabling switching of said packets
to be controlled, and a second identifier referred to as the "virtual path
identifier (VPI) enabling distribution of said packets to be controlled in
said public network;
said method being characterized in that two tasks are assigned to said
virtual path identifier, namely:
controlling distribution of the packet while it is travelling over said
public network; and
managing a routing mechanism for routing the packet while it is travelling
over one of said private sub-networks;
in that, each time a packet is transferred from one of said private
sub-networks to said public network, routing data contained in said
virtual path identifier is shifted to a reserved portion of said virtual
channel identifier, and is replaced by distribution control data;
and in that, each time a packet is transferred from said public network to
one of said private sub-networks, said distribution control data is
erased, and said self-routing data contained in said reserved portion of
said virtual channel identifier is shifted to said virtual path
identifier.
2. A method according to claim 1, characterized in that:
said virtual path identifier is constituted by 12 bits, the four most
significant bits being forced to a predetermined value; and
said virtual channel identifier is constituted by 16 bits, the eight most
significant bits being reserved for storing said self-routing data.
3. A method according to claim 1, characterized in that said mutually
distant sites comprise a main site and at least two secondary sites, and
in that any call between two of said secondary sites goes via said main
site.
4. A method according to claim 1, characterized in that the operations of
modifying the contents of said virtual path identifier are performed
physically in dedicated interface equipment on each site.
5. A method according claim 1, characterized in that each private
sub-network includes:
a plurality of switching nodes connected together in pairs via at least one
data interchange medium, each of said nodes serving a respective
concentrator, each concentrator managing at least one data processing
terminal capable of transmitting and/or receiving data packets; and
an interface module providing the connection between said private
sub-network and said public network;
and in that said method includes the following steps:
a data packet is transmitted by a transmitter terminal to its associated
concentrator, referred to as the "transmitter" concentrator, and on to a
receiver terminal, said packet carrying a local VCI (VC-La) and a local
VPI (VP-La) allocated by said transmitter terminal for the duration of a
call;
said local VCI is translated in said transmitter concentrator into a VCI
(VC-Sa) allocated by said receiver terminal for the duration of a call;
said local VPI is translated in said transmitter concentrator into a VPI
(VP-Sa) allocated permanently to said receiver terminal;
if said receiver terminal belongs to the same sub-network as said
transmitter terminal:
said packet is routed to the "receiver" concentrator associated with said
receiver terminal, via said switching nodes, by analyzing said VPI
(VP-Sa); and
the packet is routed to said receiver terminal by analyzing said VCI
(VC-Sa) in said receiver concentrator; and
if said receiver terminal belongs to another sub-network:
said packet is routed to the interface module of the sub-network of said
transmitter terminal, via said switching nodes, by analyzing said VPI
(VP-Sa);
in said interface module, the contents of said VPI (VP-Sa) are shifted to a
reserved portion of the VCI field, and management information (VP-Bx)
concerning managing the distribution over the public network is written in
the VPI field;
said interface module transmits said packet to the sub-network of said
receiver terminal via said public network;
said packet is received in the interface module of the sub-network of said
receiver terminal;
in said interface module, the contents of said reserved portion of the VCI
field are shifted to said VPI (VP-Sa);
said packet is routed to the "receiver" concentrator associated with said
receiver terminal via said switching nodes, by analyzing said VPI (VP-Sa);
and
said packet is routed to said receiver terminal, by analyzing said VCI
(VC-Sa) in said receiver concentrator.
6. A method according to claim 1, characterized in that each private
sub-network includes a plurality of switching nodes connected together in
pairs via at least one data interchange medium, and further characterized
in that said routing data placed in said virtual path identifier is an
identifier designating at least one receiver terminal, each of said
terminals having an identifier which is specific to it in a given
sub-network; wherein each switching node is associated with:
a memory containing at least one group of at least one identifier
corresponding to a set of at least one terminal associated with said node;
and
a switching matrix associating at least one input with at least two
outputs, namely;
at least one first output corresponding to at least one of said stations
associated with said node; and
at least one second output corresponding to the following node on said
private sub-network;
said method further characterized in that each matrix receiving a packet
systematically selects said second output of the switching matrix if the
identifier of the packet does not correspond to any of the identifiers
contained in the memory, and said first output otherwise.
7. A method according to claim 6, characterized in that at least one of
said sub-networks includes at least two switching units, each switching
unit including a plurality of switching nodes interconnected in pairs via
at least one data interchange medium, and at least one link node for
linking to another switching unit, via a link bridge;
in that said identifier comprises two portions:
a first portion designating the destination switching unit; and
a second portion designating the destination terminal in said destination
switching unit;
and in that, in each link node, the following steps are performed:
said first portion is analyzed; and
if the packet is to be transmitted via said link node to another switching
unit, said first portion is modified accordingly, and said packet is
transmitted.
8. A method according to claim 7, characterized in that each switching unit
uses the same set of identifiers, corresponding to said second portion,
and in that a predetermined value is added to said identifiers, the
predetermined value corresponding to said first portion, to designate the
destination switching unit.
9. A method according to claim 7, characterized in that it provides at
least one identifier enabling the packet in question to be broadcast to at
least two receiver terminals.
10. A multi-site data packet communications network, the network being of
the type including at least two mutually distant sites, each of which is
organized in private sub-networks including a plurality of terminals that
are capable of transmitting and/or receiving packets and that are
interconnected via a distributed public network; each of said packets
comprising a header and a data field, said header containing at least two
identifiers defining two access hierarchy levels as seen from said public
network, namely a first identifier referred to as the "virtual channel
identifier" (VCI) enabling switching of said packets to be controlled, and
a second identifier referred to as the "virtual path identifier" (VPI)
enabling distribution of said packets to be controlled in said public
network; said VPI controlling distribution of the packet while it is
travelling over said public network and managing a routing mechanism for
routing the packet while it is travelling over one of said private
sub-networks, wherein each time a packet is transferred from one of said
private sub-networks to said public network, routing data contained in
said virtual path identifier is shifted to a reserved portion of said
virtual channel identifier, and is replaced by distribution control data,
wherein each time a packet is transferred from said public network to one
of said private sub-networks, said distribution control data is erased,
and said self-routing data contained in said reserved portion of said
virtual channel identifier is shifted to said virtual path identifier, and
wherein at least one of said sub-networks includes at least two switching
units, each switching unit including a plurality of switching nodes
interconnected in pairs via at least one data interchange medium, said
network being further characterized in that each of said switching nodes
includes a switching matrix associated with a memory for storing a set of
identifiers.
11. A network according to claim 10, wherein each of said nodes serves a
respective concentrator, each concentrator managing at least one data
processing terminal capable of transmitting and/or receiving data packets,
said network bring further characterized in that said switching matrices
are similar to those used in the concentrators of said network.
12. A network according to claim 10, characterized in that said memories
include a fixed addressing table.
13. A network according to claim 10, characterized in that at least some of
said switching units are organized using topography having two
counter-rotary rings.
14. An interface module for providing connection between a private
sub-network and a distributed public network, in a multi-site
data-interchange network for data organized in data packets, the network
being of the type including at least two mutually distant sites, each of
which is organized in private sub-networks including a plurality of
terminals that are capable of transmitting and/or receiving packets, and
that are interconnected via said public network; each of said packets
comprising a header and a data field, said header containing at least two
identifiers defining two access hierarchy levels as seen from said public
network, namely a first identifier referred to as the "virtual channel
identifier" (VCI) enabling switching of said packets to be controlled, and
a second identifier referred to as the "virtual path identifier" (VPI)
enabling distribution of said packets to be controlled in said public
network; said virtual path identifier controlling distribution of the
packet while it is travelling over said public network and managing a
routing mechanism for routing the packet while it is travelling over one
of said private sub-networks, said module including means, each time a
packet is transferred from one of said private sub-networks to said public
network, for shifting routing data contained in said virtual path
identifier to a reserved portion of said virtual channel identifier, and
for replacing said routing date with distribution control data; and means,
each time a packet is transferred from said public network to one of said
private sub-networks, for erasing said distribution control data and
shifting said self-routing data contained in said reserved portion of said
virtual channel identifier to said virtual path identifier.
Description
BACKGROUND OF THE INVENTION
The invention generally relates to data interchange in structured
communications systems, in which calls are made by interchanging data
packets. A particular field of application of the invention is that of
systems implementing the ATM (Asychronous Transfer Mode) technique, in
which the interchanged packets are of fixed size, and are referred to as
"cells".
More precisely, the invention concerns routing cells (or more generally
packets) in a private network, in particular in a network made up of a
plurality of mutually distant sites interconnected via a public network.
Conventionally, a packet comprises at least two fields: a header and a data
field. The data field contains data that is useful to the call. The header
contains, in particular, routing data enabling the packet to be
transferred from the transmitter to its destination via various nodes
making up the communications network. The header has a predefined format
so that it can be interpreted by all of the nodes of the network, and it
must be as short as possible, so as to leave as much room as possible
available for the data that is useful to the application.
Furthermore, it is known that private networks and public networks have
different requirements in terms of routing data, if only because of the
much greater number of calls handled by a public network.
The ATM technique provides a two-level hierarchical network organization,
namely comprising virtual paths (VP) and virtual channels (VC). Generally,
the paths are set up and managed semi-permanently, as are the transmission
resources, under the control of mechanisms for administering the public
network, whereas the channels are set up under the control of mechanisms
for processing calls and connections.
In other words, the paths, which are identified in the header of each cell
by a virtual path identifier (VPI) field are managed by the public network
only, at distributors. The channels are identified by a virtual channel
identifier (VCI) field enabling the cell switching to be managed.
In a private network, only the virtual channel concept is implemented.
Therefore, it can be understood that the VPI field is unnecessary in the
private network, but that it becomes necessary as soon as a call uses the
public network.
A general object of the invention is to provide a method of optimizing use
of the bits forming the header of a packet, in particular for routing the
packet to its destination.
In this way, an object of the invention is to provide such a method that
makes it possible to re-use, at private level, a header field that is
provided to be used over the public network only, such as the VPI field of
the ATM standard, while guaranteeing the continued existence of this field
whenever it is necessary to transfer a packet to the public network.
In other words, an object of the invention is to provide such a method that
enables the same field of the header of a packet to have two uses,
depending on whether the packet is in a public network or in a private
network, and that restores the field each time the packet passes from the
private network to the public network, and vice versa.
In other words, an object of the invention is to broaden the possibilities
for use of a packet header, without increasing the size of the header, or
modifying its structure too significantly.
In this way, a particular object of the invention is to provide such a
method that enables packets to be routed to a large number of stations. In
particular, an object of the invention is to provide such a method that,
in systems using the ATM technique, makes it possible to address a higher
number of stations than known methods, whether using conventional routing
via the VCI field, or the "self-routing" technique.
In accordance with the ATM standard, each cell entering a switching matrix
of the network undergoes processing which consists inter alia in reading
its header (more precisely the contents of the VCI field and/or the VPI
field) so as to determine that direction in which it must be switched at
the output.
Such information is associated with the number of the input multiplex via
which the cell arrived so as to constitute the address of the translation
memory of the matrix. The read data is constituted firstly by the
number(s) of the multiplexes via which the cell is to be transmitted at
the output, and secondly by the new VCI (or VPI) code(s). The first
operation is referred to as "routing", and the second is referred to as
"translation".
An ATM multiplex is designed to convey a large number of
simultaneously-routed calls. These calls are routed via virtual
connections identified by the VCI fields. On any given multiplex, all of
the virtual connections that carry the same VCI belong to the same call.
Since the standard defines a VCI field of 16 bits, the maximum number of
virtual connections routed simultaneously on the same multiplex is about
65,000, which corresponds to a translation memory size of 16.times.65,000
words of (a minimum of) 32 bits for an ATM matrix having 16 incoming
multiplexes and 16 outgoing multiplexes.
In addition to the drawback of requiring a large-sized translation memory,
that is therefore costly because of the silicon area required, that
routing technique implements message interchange (marking) with a control
processor during set-up and clearing-down of each connection. Such
interchange is necessary with all of the switching matrices situated on
the route to be travelled by the cells of the virtual connection to be set
up. The number of switching layers that interconnect the terminals between
which a call is to be set up may, in some cases, be large because this
number depends on the size of the site, on the route followed, and on the
topology of the network.
Use of large-sized (e.g. 16.times.16) matrices is quite compatible with
these requirements insofar as:
a) the ratio between the required translation memory volume and the
hardware taken up by the matrix (one or two cards) remains very
advantageous; and
b) the marking stages concern only a limited number of items of switching
equipment.
Unfortunately, as soon as various reasons (topology, modularity,
maintenance, expandability, etc.) justify implementation of small-sized
(or even single-component) switching equipment, the question of reducing
the sizes of translation memories becomes relevant.
The self-routing technique is not governed by a standard but it is
nevertheless widely used in widely differing forms. In principle, it
consists in explicitly coding, in a field of a header of a cell, the list
of the numbers of the links via which the cell is to be routed.
The drawback with that technique, which may be referred to as the
"consumable label" technique, is that it requires a field having a length
that is proportional to the number of switching layers via which the cell
is to pass: it must therefore be reserved for small-sized networks.
By way of example, a field of 16 bits makes it possible to code the
successive passes through only 4 16.times.16 matrices (4 bits for coding
one output multiplex from 16). This drawback can however be overcome by
means of various solutions.
A first solution consists in using the 24 bits represented by the VPI field
and the VCI field together. That suffers from two drawbacks, namely it is
necessary to provide a specific routing analysis logic circuit in each of
the matrices, and it is impossible to superpose a second VCI or VPI
routing level.
A second solution consists in encapsulating the 5-byte (standardized size)
header with additional bytes, e.g. 3 bytes.
Another solution uses a compound approach: self-routing then translation
(i.e. updating the self-routing label), then self-routing, etc.
The cell self-routing technique eliminates the translation memory marking
stage, which becomes unnecessary. It also makes it possible if not to
simulate the translation memories, at least to reduce their capacities
significantly.
Another advantage of self-routing is that it offers, quite cheaply, the
possibility of using back-up routes rapidly for transferring data in the
event of local breakdowns or of traffic congestion. It is merely necessary
to change the contents of the self-routing address in the transmitter
terminal in order for the cells of the virtual connection to be forwarded
over a different route that can by-pass the obstacle; naturally, after the
self-routing address has been stored in the cell transmitter equipment.
Another object of the invention is to provide a method offering the
advantages of self-routing, without suffering from the drawbacks thereof.
In particular, an object of the invention is to provide such a method that
enables a network to be covered that has a size which is compatible with
the requirements of private sites, while maintaining the structural
integrity of the fields as defined in the standards, such as the VCI and
VPI fields in ATM.
Another object of the invention is to provide such a method that is
compatible with all types of network (single or double ring, bus, star,
etc.) and with all combinations of networks. In particular, an object of
the invention is to provide such a network that makes it simple to
interconnect a plurality of sub-networks, either directly or via the
public network.
Yet another object of the invention is to provide such a method that
enables small-capacity translation memories to be used, thereby making it
possible to integrate small-sized switching entities, e.g. in the form of
ASIC-type components. Using small-sized switching components offers
numerous advantages, such as modularity and mass-producibility, thereby
making it possible to optimize manufacturing, installation, and
maintenance costs, and to make operating costs linear, and the possibility
of implementing widely differing network architectures: in particular,
distribution networks can be built using centralized topologies but also
geographically distributed topologies, e.g. loops, buses, or trees.
Another object of the invention is to provide such a method that does not
require a specific self-routing mechanism, i.e. additional equipment, in
the switching matrices. In other words, an object of the invention is to
use matrices that comply with the standard, and therefore that are
compatible with use in public network multiplexing and switching
equipment.
SUMMARY OF THE INVENTION
The invention achieves these objects and others that appear below by
providing a method of forwarding data organized in data packets, in a
multi-site data-interchange network for data organized in data packets,
the network being of the type including at least two mutually distant
sites, each of which is organized in private sub-networks including a
plurality of terminals that are capable of transmitting and/or receiving
packets, and that are interconnected via a distributed public network;
each of said packets comprising a header and a data field, said header
containing at least two identifiers defining two access hierarchy levels
as seen from said public network, namely a first identifier referred to as
the "virtual channel identifier" (VCI) enabling switching of said packets
to be controlled, and a second identifier referred to as the "virtual path
identifier (VPI) enabling distribution of said packets to be controlled in
said public network;
in which method two tasks are assigned to said virtual path identifier,
namely:
controlling distribution of the packet while it is travelling over said
public network; and
managing a routing mechanism for routing the packet while it is travelling
over one of said private sub-networks;
in which method, each time a packet is transferred from one of said private
sub-networks to said public network, routing data contained in said
virtual path identifier is shifted to a reserved portion of said virtual
channel identifier, and is replaced by distribution control data;
and in which method, each time a packet is transferred from said public
network to one of said private sub-networks, said distribution control
data is erased, and said self-routing data contained in said reserved
portion of said virtual channel identifier is shifted to said virtual path
identifier.
In other words, a zone of the header of each cell, or packet, and more
precisely of the VCI, is reserved so as to serve as a temporary storage
zone for temporarily storing the "private" contents of the VPI, so long as
the cell is travelling over the distributed public network. Each time the
cell is transferred from the public network to the private network, and
vice versa, the VPI is modified so as to correspond to the task that is
assigned to it.
In this way, addressing over the network is facilitated because both fields
(VPI and VCI) can be used. For example, this makes it possible to define
two addressing levels in the private network;
In a preferred implementation of the invention, said packets are ATM cells,
and:
said virtual path identifier is constituted by 12 bits, the four most
significant bits being forced to a predetermined value (in other words,
only the eight least significant bits are used); and
said virtual channel identifier is constituted by 16 bits, the eight most
significant bits being reserved for storing said self-routing data.
It is these eight reserved bits that enable the method of the invention to
be implemented.
Advantageously, said mutually distant sites comprise a main site and at
least two secondary sites, any call between two of said secondary sites
going via said main site.
As appears more clearly below, this technique makes it possible to reduce
the number of permanent paths to be set up in the public network.
Preferably, the operations of modifying the contents of said virtual path
identifier are performed physically in dedicated interface equipment on
each site.
In a preferred implementation of the invention each private sub-network
includes:
a plurality of switching nodes connected together in pairs via at least one
data interchange medium, each of said nodes serving a respective
concentrator, each concentrator managing at least one data processing
terminal capable of transmitting and/or receiving data packets; and
an interface module providing the connection between said private
sub-network and said public network;
and the method includes the following steps:
a data packet is transmitted by a transmitter terminal to its associated
concentrator, referred to as the "transmitter" concentrator, and on to a
receiver terminal, said packet carrying a local VCI (VC-La) and a local
VPI (VP-La) allocated by said transmitter terminal for the duration of a
call;
said local VCI is translated in said transmitter concentrator into a VCI
(VC-Sa) allocated by said receiver terminal for the duration of a call;
said local VPI is translated in said transmitter concentrator into a VPI
(VP-Sa) allocated permanently to said receiver terminal;
if said receiver terminal belongs to the same sub-network as said
transmitter terminal:
said packet is routed to the "receiver" concentrator associated with said
receiver terminal, via said switching nodes, by analyzing said VPI; and
the packet is routed to said receiver terminal by analyzing said VCI in
said receiver concentrator; and
if said receiver terminal belongs to another sub-network:
said packet is routed to the interface module of the sub-network of said
transmitter terminal, via said switching nodes, by analyzing said VPI;
in said interface module, the contents of said VPI (VP-Sa) are shifted to a
reserved portion of the VCI field, and management information (VP-Bx)
concerning managing the distribution over the public network is written in
the VPI field;
said interface module transmits said packet to the sub-network of said
receiver terminal via said public network;
said packet is received in the interface module of the sub-network of said
receiver terminal;
in said interface module, the contents of said reserved portion of the VCI
field are shifted to said VPI (VP-Sa);
said packet is routed to the "receiver" concentrator associated with said
receiver terminal via said switching nodes, by analyzing said VPI (VP-Sa);
and
said packet is routed to said receiver terminal, by analyzing said VCI in
said receiver concentrator.
Advantageously, said routing data placed in said virtual path identifier is
an identifier designating at least one receiver terminal, each of said
terminals having an identifier which is specific to it in a given
sub-network;
each switching node is associated with:
a memory containing at least one group of at least one identifier
corresponding to a set of at least one terminal associated with said node;
and
a switching matrix associating at least one input with at least two
outputs, namely:
at least one first output corresponding to at least one of said stations
associated with said node; and
at least one second output corresponding to the following node on said
switching unit;
and each matrix receiving a packet systematically selects one of said
second outputs of the switching matrix if the identifier of the packet
does not correspond to any of the identifiers contained in the memory, and
one of said first outputs otherwise.
This routing technique offers several advantages. In particular, it makes
it simple to address a large number of nodes. It is based on using
matrices that are identical to those used elsewhere in ATM networks.
Finally, it requires no action to be taken on the contents of the cells in
each node, and it merely requires switching to be performed.
In a preferred implementation, at least one of said sub-networks includes
at least two switching units, each switching unit including a plurality of
switching nodes interconnected in pairs via at least one data interchange
medium, and at least one link node for linking to another switching unit,
via a link bridge;
and said identifier comprises two portions:
a first portion designating the destination switching unit; and
a second portion designating the destination terminal in said destination
switching unit;
so that, in each link node, the following steps are performed:
said first portion is analyzed; and
if the packet is to be transmitted via said link node to another switching
unit, said first portion is modified accordingly, and said packet is
transmitted.
In which case, advantageously, each switching unit uses the same set of
identifiers, corresponding to said second portion, and a predetermined
value is added to said identifiers, the predetermined value corresponding
to said first portion, to designate the destination switching unit.
Preferably, the method provides at least one identifier enabling the packet
in question to be broadcast to at least two receiver terminals. In this
way, it is possible to broadcast only, although it is generally considered
that the self-routing technique is not compatible with broadcasting only.
Naturally, the invention also provides data packet communications networks
implementing the above-described method.
Advantageously, in such a network, said switching matrices are similar to
those used in the concentrators of said network, and the corresponding
memories include a fixed addressing table.
In a preferred embodiment, at least some of said switching units of the
network are organized using topography having two counter-rotary rings.
Finally, the invention also specifically provides the interface modules
providing connection between a private sub-network and the public network,
as implemented by the above-described method.
BRIEF DESCRIPTION OF THE DRAWING
Other characteristics and advantages of the invention appear more clearly
on reading the following description of a preferred implementation of the
invention given by way of non-limiting example, and with reference to the
accompanying drawings, in which:
FIG. 1 shows a multi-site network of the invention, comprising one main
site and three remote sites interconnected via the distributed public
network;
FIG. 2 shows the various types of connection that it must be possible to
set up in the network shown in FIG. 1; FIG. 3 shows the beginning of the
header of an ATM cell, in accordance with the method of the invention,
with a shift zone enabling the VPI field to have two uses;
FIG. 4 is an overall flow-chart of the method of the invention, using the
cells shown in FIG. 3;
FIGS. 5 to 8 show the FIG. 4 method, in a particular implementation, and
more particularly:
FIG. 5 concerns cell interchange inside the main site;
FIG. 6 shows the case of two connections set up simultaneously from the
main site to the same terminal of a remote unit;
FIG. 7 applies to the case of two connections to the same terminal of the
main site from two remote terminals; and
FIG. 8 shows the case of a connection between two terminals belonging to
different remote units, via the main site;
FIG. 9 illustrates the architecture of a medium-sized network design to
which the mechanism of the invention is applied;
FIG. 10 is an example of a private site (main or remote) of a network as
shown in FIG. 1, the site being composed of three distinct switching
units;
FIG. 11 shows cell transit between two switching units of the site shown in
FIG. 10; and
FIG. 12 shows cell broadcast in the switching units of the site shown in
FIG. 10.
DETAILED DESCRIPTION OF THE INVENTION
The invention concerns private local area networks such as company sites,
campuses, metropolitan area networks (MAN), etc., and more particularly
multi-site networks in which a plurality of sites are interconnected via a
public network.
It applies to network architectures based on packet interchange, in
particular those based on the ATM technique, which is described in
particular in special editions 144 and 145 of "1'Echo des Recherches"
printed in the second and third quarters of 1991 by the "Centre National
d'Etudes des Telecommunications" and the "Ecole Nationale Superieure des
Telecommunications".
An example of such a network to which the invention applies is shown in
FIG. 1. The network in question is made up of a set of switching units
(UCX) 11 that may be mutually meshed (a single UCX is shown in FIG. 1).
The terminal equipment 12.sub.1 to 12.sub.N (workstations, multimedia
terminals, bridges, routers, public network access equipment, control and
administration members, etc.) is connected to peripheral connection
equipment (EPR) 13.sub.1 to 13.sub.M. The generic term "terminal" is used
below to designate any type of terminal equipment.
An EPR 13.sub.i is capable of concentrating (or multiplexing) an arbitrary
number of terminals 12.sub.j. This number is limited by the data-rates of
the terminal links. By way of example, it may be set at 16 (this has no
incidence on the provisions of the present invention).
The UCXs and the EPRs use a single type of ATM switching element: a
4.times.4 matrix (MCX) that is capable of switching cells on the basis of
analysis of an 8-bit VPI field or of an 8-bit VCI field. The choice is
determined by the hardware or software configuration on switching on the
module. As described below, other modules enable action to be taken on the
fields of the headers of the cells without performing switching functions.
The installation 11 is connected to the public network via equipment
referred to as a "public network connection interface" (IRR) 14.
A multi-site private network includes certain items of equipment 15 that
are remote but that can be interconnected via the distributed public
network (RPB) 16, such equipment being located at distances that can
exceed the size of a national network. Such items of equipment may be:
terminals 12.sub.N ;
terminal concentrators 13.sub.M ; or
switches.
In such a private network, only the main site 11 performs the
above-mentioned procedures. In other words, the public network 16 is
completely transparent to signalling messages between the peripheral
equipment and the control members situated on the main site. Everything
takes place as if the RPB 16 did not exist. Indeed, an object of the
invention is to cause the cells to be adapted before they pass through the
RPB, and then to be restored after passing therethrough.
The architecture in question therefore includes the following entities:
one main site 11 constituted by a connection unit (set of ATM switches
17.sub.1 to 17.sub.L) to which the local EPRs 13.sub.1 to 13.sub.M and an
IRR 14 are connected; the local EPRs (EPR-L) connect peripheral equipment
12.sub.1 to 12.sub.N : multimedia terminals, control members 18
(administration and call processing), access bridges providing access to a
PABX 19, to existing local area networks; and
remote units 110.sub.1 to 110.sub.3 constituted by one or more
concentrators EPR-D 13.sub.M ; an EPR-D is designed to connect the same
peripheral equipment as an EPR-L 13.sub.4 except for control members
(thereby taking away from it any autonomy with respect to call
management).
The transmission links between the various entities of the multi-site
installation and the RPB are based on the SDH standard. The links are of
the virtual type, they are referred to as "virtual paths" (VP), and they
are identified by VPI codes carried by the headers of the ATM cells.
The VPs are reserved links characterized by a peak data-rate that must not
be exceeded by the user. The main site sees each remote unit via a VP:
each VP is identified by a VPI code and by a peak data-rate that may
differ from one unit to another.
Each remote unit sees the main site via a single VP that is also
characterized by a peak data-rate.
The VPs are allocated and administered by the operator of the public
network, and the private user is charged on the basis of the VPs that said
user rents from the operator.
A multi-site network may be modified at any time by removing a unit, or by
adding additional units. Naturally, these operations take place under the
control of the public operator. Such modifications also involve action
being taken on the administration of the multi-site private network, but
they remain simple to manage.
The maximum number of remote units depends essentially on the data-rate
resources that the operator can supply to the user, and also on the level
of investment that the user is ready to make.
To summarize, the multi-site network of the invention is similar to a
"master-slave" type architecture, the master being represented by the main
site, and the slaves being represented by the remote units. In particular,
this structure offers the following advantages:
it lends itself very well to private installations having one main center
surrounded by small geographically dispersed installations: offices,
subsidiaries, branches, etc.;
with respect to managing the VPs of the distributed network, it can be
observed that the virtual topology is of the star type: there are as many
VP links as there are remote entities, the links converging towards the
access multiplex of the main site; unlike a multi-site network with
autonomous sites for which the various sites must be meshed, which
represents a more costly approach to rental of rented VPs;
calls between remote units must be switched at main site level, whereas
calls between peripheral equipment connected to the same concentrator
(EPR-D) may be established locally, with only call processing being
obliged to "go up" as far as the main site; and
a remote unit possesses only a very small amount of operating autonomy, and
this gives it the advantage of being less costly with respect both to
initial investment and to management expenses.
The method of the invention must therefore make it possible to achieve
simultaneously in particular the virtual connections shown in FIG. 2:
connection 21 between two terminals connected to the main site (SP) 11 or
to a remote unit, without passing through the distributed public network
(RBP);
connection 22 between a terminal of the SP and one or more terminals
connected to the SP via the RBP;
connection 23 between a remote terminal and one or more terminals on the
SP, via the RBP; and
connection 24 between remote terminals, with the call passing twice through
the RBP.
The method of the invention aims to provide multi-site private networks as
defined above in an environment having a public network that is
distributed at VP level. The invention also concerns the technique to be
implemented to enable the distributed public network to be inserted such
that it is transparent:
to the internal signalling protocols;
to the administration procedures; and
to the connections set up between the various terminals.
The general principle consists in acting via hardware on the fields of the
headers of the cells so that the headers comply with the routing:
in the public network when they leave either the main site or a remote
unit; and
in the main site or in a remote unit when they leave the RPB.
The ATM standard provides that the header of each cell includes in
particular, as shown in FIG. 3 (showing the beginning of the header of a
cell) a 12-bit VPI field 31, followed by a 16-bit VCI field 32.
In accordance with the invention, the VPI and VCI fields are subjected to
particular constraints:
the connections are identified at the terminal equipment by analyzing the
VCI field 32 which must be limited to 8 bits, the most significant
eight-bit byte being coded to zero; and
the 4 most significant bits of the VPI field 31 are coded to zero.
These constraints make it possible to provide a shift zone 33 used for
implementing the method of the invention, as shown in FIG. 4 which gives
the main outline of the method.
The method comprises the following steps:
a cell is transmitted 41 with a local (or private) VCI and a local (or
private) VPI, i.e. that can be used on the private network;
the cell is forwarded 42 over the portion of private network from the
transmitter to a connection node for providing connection to the
distributed public network;
in this connection node, the private VPI is shifted to the zone 33 reserved
for that purpose, and a public VPI is written (43);
the cell is forwarded 44 (distributed) over the public network, by means of
the public VPI;
on reception in a connection node of the portion of the private network
that contains the destination, the public VPI is erased, and the private
VPI stored in the zone 33 is restored by being shifted (45); and
the cell is forwarded 46 to its destination.
In the preferred implementation of the invention that is described in more
detail below, forwarding and routing the cells implements both the VP
level and the VC level:
in the UCXs, routing is performed by analyzing the contents of the VPI
fields of the cells, using a technique that can be likened to the
self-routing mode insofar as the code contained in the VPI field
identifies a physical destination address; and
in the EPRs, switching is performed conventionally by analyzing the VCI
field, but it differs from the switching performed by the VC switches in
the public network insofar as the contents of VPI fields are not taken
into account, which means that in this case, at the input of an EPR, all
of the VCI codes are different on each multiplex.
In particular, the technique offers the following advantages:
marking the set-up connections call-by-call is performed only at the ends
(in the EPRs), which offers advantages in terms of protocol simplicity and
rapidity, and improved back-up, compared with procedures requiring marking
to be performed in all of the switching matrices that exist on a link
between the terminals to be interconnected virtually; and
since the connections are identified by VCI chosen by the destination EPRs,
the number of connections that can be set up simultaneously is very large.
Firstly, cell routing within the main site is described with reference to
FIG. 5.
Consideration is given to a cell 51 travelling from a transmission EPR 52
(EPR-e) to a reception EPR 53 (EPR-r) via the switching network 64.
At the source end, the cell 51 carries a local VPI 55 (VP-La) allocated by
the EPR-e 52 on setting up the connection. It carries a local VCI (VC-La)
also allocated by the EPR-e.
In the first switching stage 57, the cell is switched to a common multiplex
with translation into a VCI 18 being performed, which VCI has been
allocated by the destination EPR-r 53. No further change takes place in
this VC-Sa until it arrives at its destination: in this way conflict is
avoided insofar as it is always the reception end which allocates the VCI
codes to each connection.
It is recalled that VP-La, VC-La and VC-Sa are allocated for the duration
of the call only.
In a second stage 59 of the EPR-e 52, the VPI code is translated into a
VP-Sa code 510 which is specific to the target direction, i.e. that of the
EPR-r. This code is permanent and it is the same for any source calling
the same destination.
Switching in the switching network 54 is performed by analyzing the
contents of the VPI field 510. It is recalled that all of the virtual
paths are set up permanently inside the network, a target direction is
identified by a VPI code (a second code identifying a second route is
provided for back-up or traffic-distribution reasons). This type of
routing is close to the self-routing technique.
All of the cells coming from different sources and going to the same EPR-r
53 are multiplexed on the EPR-r access multiplex. They all carry the same
VPI code VP-S' a 512, and only the rVCI field 513 makes it possible to
identify that connection to which they belong.
By analyzing the code 513 in the switching modules 514 of the EPR-r, it is
possible to switch them to the destination output multiplex. It is the
terminal which demultiplexes any connections addressed to it
simultaneously, such demultiplexing also being performed by analyzing the
VCI field.
Naturally, the process is absolutely identical for the other direction of
the connections, with the EPR-e then acting as the EPR-r, and the EPR-r
then acting as the EPR-e. However, it should be noted that the VCI and VPI
codes are not allocated symmetrically at the ends, i.e. for a both-way
call which implements two virtual connections (one per direction), for a
given end, the outgoing and incoming VPI codes and VCI codes may differ.
The invention is particularly advantageous when a remote unit is to be
connected to the main site, or to another remote unit. A description is
given below of the operations performed on the fields of the headers of
cells in order to prepare them for transiting via the distributed public
network. These operations are entirely performed by hardware in specific
modules contained in interface equipment: the IRR for the interface
between the main site and the RPB, and the EPR-D for the remote units.
FIG. 6 concerns the case of two connections set up simultaneously towards
the same unit 61. It shows the contents of the various fields of the
header for each reference point of the link between the transmitter
terminal and the receiver terminal.
The novelty lies essentially in the passage through the IRR 65. The
functions performed are as follows:
the private VPI code (referenced VP-Sa) 66 is translated into a VPI
(referenced VP-Bx) 67 reserved for the distributed public network; and
the private code VP-a 66 is simultaneously transferred to the place 68 of
the most significant byte of the VCI field.
This operation is performed in the hardware main module of the IRR 65
referred to as the "access to the distributed network from the main site"
(ARBP).
It should be noted that this operation is made possible only because, by
assumption, the multi-site installation uses only the 8 least significant
bits of the VCI code.
The RPB network 69 preforms the routing on the basis of the VPIs, the VPI
codes generally being translated, it is assumed that the cell arrives in
the remote unit with the VPI referenced (VP-By) 610.
As the cell passes through a module 611 in the EPR-D 61, the private VPI
code 612 is restored by shifting the contents of the most significant byte
of the VCI field to the VPI field. This hardware module 611 is referred to
as the "access to a remote unit from the distributed network" (ADRP).
It should be noted that the cells that have different origins (EPR-Lx 62
and EPR-Ly 63) carry the same VPI codes (VP-Bx) 67 and (VP-By) 610 at RPB
level because they are conveyed by the same VP in the distributed public
network 69.
Naturally, however, the VC-La 613 and 614 of each EPR-L 62 and 63 are
translated in different ways (VC-Sa 615 and VC-Sb 616) so as to identify
each connection.
FIG. 7 shows the routes taken by cells from 2 distinct EPR-Ds (EPR-Dx 71
and EPR-Dy 72) to an EPR-L 73 of the main site 74. Compared with the
above-described case, it can be observed that there are no IRRs in the
EPR-Ds 71 and 72, and that they must perform the functions of adapting the
cells to the public network 75.
In this case, such adaptation involves:
translating a local VPI code 76, 77 (referenced VP-Lr) into a VPI code of
the RPB (referenced VP-By 78 or VP-Bw 79 depending on the EPR in
question); and
simultaneously inserting the VPI code (referenced VPS-rd) that is to be
used in the main site to forward the cell to the destination EPR-L into
the most significant byte of the VPI field.
This operation is performed in a hardware module (710, 711 that is quite
similar to module ARBP, and that is referred to as the "access to the
distributed network from a remote unit" (ARBD).
As the cells pass through a module 712 in the IRR of the main site 74, the
contents of the headers of the cells are restored so that said cells are
switched in the main site towards the target EPR-L.
This involves:
removing the RPB routing VPI code (in this example VP-Bx 713 or VP-Bz 714)
and replacing it with the private VPI code (VP-Srd in this example); said
private VPI code may itself be translated again (716) inside the main
site; and
writing the most significant byte 717 in the VCI field with the value 0.
A connection between two remote units is now described with reference to
FIG. 8.
This type of connection represents an important element of the invention. A
connection between two remote units 81 and 82 necessarily goes via the
main site 83 and passes twice through the RPB 85. More precisely,
switching takes place in the IRR so as not to burden the resources of the
main site 83 unnecessarily.
The other alternative which consists in setting up permanent VP-type paths
in the RPB between each pair of remote units has been intentionally
rejected insofar as it requires a very large number of reserved VPs (full
interconnection of N units taken in pairs requires N(N-1)/2 VPs).
FIG. 8 shows the routes taken by the cells between 2 remote units (EPR-Di
81 and EPR-Dk 82).
It can be observed that, in this example, the headers of the cells are
acted on twice at main site level: firstly when they arrive in the IRR,
and secondly when they leave it.
It is as if both part-halves of the above-described connections were placed
end-to-end.
In this way, the headers undergo the following transformations: in the
direction going from EPR-Dj 81 to EPR-Dk 82 (the process in the other
direction is symmetrical, and is therefore not described in detail):
the cell 86 is transmitted by a transmitter terminal to EPR-Dj with a local
VP-La 87 and a local VC-La 88;
the VP-La 87 is translated into a private VPI VP-Sk 89 and it is then
shifted to the most significant byte of the VCI by the ARBD module of the
EPR-Dj 81;
the VC-La 88 is translated into a private VCI VC-Sa 810 by the ARBD module
of the EPR-Dj 81;
the VPI-La 87 is translated into a public VPI VP-By 811;
the cell 812 is transmitted over the RPB 85 to the IRR of the main site 83;
the APRB of the IRR shifts the private VPI VPI-Sk 813 to the VPI field;
the private VPI VP-Sk 813 is shifted to the most significant byte of the
VCI by the ARPB module of the IRR 84;
the VPI-Sk 813 is translated into a public VPI VP-Bz 814;
the cell 815 is transmitted over the RPB 85 to the EPR-Dk-82;
the ADRB of the EPR-Dk shifts the private VPI VP-Sk 816 to the VPI field;
and
the cell 817 is forwarded to the destination terminal.
The invention also proposes a particularly advantageous mode of forwarding
cells in a main site or in a remote unit by using the VCI and the private
VPI released by the above-described method.
This technique is based on a mechanism implementing:
1. self-routing by using the contents of the VPI field in the center of the
system (the switching network); and
2. VCI routing by using the 8 least significant bits only (from a field of
16 bits) in the multiplexing and demultiplexing peripheral connection
equipment.
In this way, a cell transmitted by a transmitter terminal to a destination
terminal is routed successively:
by VCI (conventionally) in the transmitter-end connection equipment;
by self-routing (using the VPI field) in the switching center; and
by VCI again, in the reception-end connection equipment.
The switching entities (matrices) used are identical in both levels. The
translation memory is the same: in self-routing, said memory is addressed
by the contents of the VPI field of the header, and in VCI routing, it is
addressed by the contents of the VCI field. The choice of operating mode
of a matrix may be made in two ways:
implicitly by programming in the matrix on initialization; or
explicitly by positioning a bit in the header of each cell and analyzing
the bit in the matrix.
Since both the VCI field and the VPI field have the same length (8 bits),
the translation memory (having a capacity of 1,024 words) is very easy to
integrate.
With this technique, in order to set up a virtual connection, it is
necessary to manage 2 identifiers for a given direction:
the self-routing address (contained in the VPI field) calculated in the
control member on the basis of the physical identifiers of the terminals
between which the call is to be set up; the calculation algorithm is very
simple and is described below; and
the VCI code allocated by the destination end, and chosen from those of the
available 256 codes which are unused.
The process of setting up the virtual path is then performed in each of the
ends to which the calling terminal and the called terminal are connected.
Such marking is performed in the form of a very small number of processor
interchange operations insofar as they remain localized at the periphery:
the number of switching matrices concerned is therefore very small. In
particular, no action is necessary at the matrices in the switching
system.
The call is set up once all of the virtual connections that make up said
call have been set up: a single connection if the call is one-way, two
connections if it is a both-way call, and several connections for more
complex calls which may or may not be symmetrical.
The above-described self-routing principle using the VPI field requires an
initialization stage for initializing the network, and more precisely the
switching portion, i.e; the portion of the network in which the cells are
self-routed. This operation is performed before the installation is put
into service, as soon as it is switched on.
It is performed under the control of a processor, and it consists in
updating the translation memories of all of the switching matrices: the
information to be loaded is output by a configuration table the contents
of which can change during the life of the system as a function of the
physical modifications to which the network is subjected: extension,
removal, re-configuration, etc. Any change in then contents of a table
causes all or part of the network to be re-initialized, thereby updating
the contents of the translation memories of all of the switching matrices
concerned. The following description of the principles as applied to a
network architecture design outlines the nature of the information to be
loaded.
The number of processors varies as a function of the size of the network.
Generally, it is accepted that each geographical zone of a given site is
served by a switch, and that this switch is under the control of a control
processor.
In order to illustrate the principles implemented, consideration is given
to a medium-sized network architecture design to which the mechanism of
the invention is applied as shown in FIG. 9. This network is physically
made up of 3 switching units (referenced UCX1 91, UCX2 92, and UCX3 93)
meshed in pairs. Each UCX to which terminal connection equipment (EPR) 94i
is connected covers a local zone of the site. The advantage of
self-routing is particularly apparent in network architectures made up of
long chains of switching equipment, and in particular in distributed
topolologies. That is why the UCXs 91, 92, 93 are loop structures having
two counter-rotary rings 96.sub.0, 96.sub.1 comprising 16 ATM nodes 95i.
FIG. 9 shows the architecture design with the various constituent elements
being referenced using referencing conventions:
for each UCX, the 16 elementary ATM switching matrices 95i (MCX) are
referenced from (A) to (S), with (A) and (J) designating bridges;
the bridge MCXs are also referenced Pxy, where x designates the number of
the UCX to which the MCX belongs, and y designates the UCX to which it is
connected.
The peripheral connection equipment 94i is referenced EPR.
It should be noted that the MCXs are 4.times.4 ATM matrices interconnected
in loops, and the multiplexes ranked 4 in the loops are not used.
The numbers of possible routes between pairs of EPR are:
2, between EPRs in the same UCX;
4, between EPRs in different UCXs with direct access; and
8, between EPRs in different UCXs with transit via the third UCX.
By using all of the possibilities, the number of routes between 2 EPRs
belonging to different UCXs may be as much as 12. It is recalled that
multiple routes constitute a very advantageous characteristic enabling the
network to withstand certain types of breakdown, while being easy for the
self-routing technique to operate.
With the self-routing label using the VPI field having a length of 8 bits,
a number of VPI codes limited to 256 means that a maximum of 2 routes can
be used regardless of the respective locations of the EPRs:
locally within a UCX: either via the 0-ring or via the 1-ring;
directly between 2 UCXs: either via the 2 0-rings or via the 2 1-rings; and
transiting via a UCX: either via the 3 0-rings or via the 3 1-rings.
The following abbreviations are defined:
the caller terminal is referenced Dr, and the called terminal is referenced
De; Dr and De are generally physical identifiers;
AcDr designates the network access point to which the EPR at the caller end
is connected;
AcDe designates the access point to which the EPR at the called end is
connected;
[X] designates the self-routing address contained in the VPI field for
routing the cells from the caller terminal to the called terminal;
[Y] designates the self-routing address for going from the called terminal
to the caller terminal;
i.sub.-- a0 designates the number of a node as seen by the 0-ring; and
i.sub.-- a1 designates the number of a node as seen by the 1-ring.
In other words:
[X] identifies the virtual path between the access point AcDr and the
access point AcDe; and
[Y] identifies the virtual path between the access point AcDe and the
access point AcDr.
The VCI codes required for peripheral switching and identifying the
connections between the terminals are referenced as follows:
In the direction going from the caller terminal to the called terminal:
(p): VCI code allocated by the called EPR for identifying the virtual
connection at the called EPR end; and
(m): VCI code allocated by the caller EPR for identifying the same virtual
connection at the caller end;
(m) being translated into (p) at the caller EPR end.
In the direction going from the called terminal to the caller terminal:
(q): VCI code allocated by the caller EPR for identifying the virtual
connection at the caller EPR end; and
(n): VCI code allocated by the called EPR for identifying the same virtual
connection at the called end;
(n) being translated into (q) at the called EPR end.
Setting up a call includes inter alia the following steps:
Managing connections at the periphery
During setting-up of the connection in the direction from the caller
terminal to the called terminal:
a) the VCI (m) is written in the translation memories of the matrices of
the caller EPR; and
b) the VCI (p) is written in the translation memories of the matrices of
the called EPR.
During clearing-down of the connection, (n) and (p) are erased from the
translation memories.
During setting-up of the connection in the direction going from the called
terminal to the caller terminal:
a) the VCI (n) is written in the translation memories of the matrices of
the called EPR; and
b) the VCI (q) is written in the translation memories of the matrices of
the caller EPR.
During clearing-down of the connection, (n) and (q) are erased from the
translation memories.
The preceding paragraph mentions 2 VCI per direction: (m) then (p) in the
go direction (caller terminal to called terminal), and (n) then (q) in the
return direction (called terminal to caller terminal).
Starting from a given EPR, a plurality of connections may be set up
simultaneously: these connections may carry the same VCIs insofar as said
VCIs are allocated by the destination EPRs without consultation
therebetween. But, in order to clear-down the connections independently
from one another, it must be possible to identify them individually in the
starting EPR: this is the task of the starting VCI allocated to connection
set-up by the transmitter EPR.
Managing connections by the control member
This concerns seeking routes and is performed by calculating self-routing
codes.
The codes are calculated on the basis of the physical identities of the
accesses to which the caller EPR and the called EPR are connected, taking
into account the image of the network that possesses the control member.
The physical identities of the accesses AcDr and AcDe are sent by means of
signalling messages interchanged by the terminals and by the control
member during setting-up of the call. This control member possesses the
physical image of the network and the memory at any time of all of the
connections set-up and of the parameters thereof: number, routes followed,
possible back-up routes, and loads on all of the multiplex links. Among
the different possible routes between the AcDr and the AcDe, the control
member makes a choice as a function of criteria of the following type:
load, distance, transit time, etc. which results in proposing self-routing
codes to each of the ends (the caller end and the called end). Each of the
ends receives at least one code, but it may receive a plurality of codes,
some of which designate back-up routes that each terminal might have cause
to use in the event of an operating anomaly: e.g. a link being cut off, or
accidental traffic overload occurring.
In the remainder of the description, when mentioned is made of codes in the
form of decimal numbers, reference is made to the tables in Appendix 1.
The following principles assume that the stage prior to initialization has
been performed, i.e. updating the translation memories of all of the MCXs
in all three UCXs, the data to be loaded complying with the contents of
the tables in Appendix 1.
Local addressing in a UCX
An EPR addresses another EPR by using one of the two VPI codes allocated to
the target EPR depending on whether the chosen route uses the 0-ring or
the 1-ring, the selection criterion being the shortest route (measured in
numbers of nodes through which the route passes) or the route which is the
least heavily loaded. The bridge nodes cannot be directly addressed.
Therefore, for a UCX having a capacity of 16 nodes including 14 EPRs, the
number of codes required is 28. These 28 codes are the same inside each
UCX (see Appendix 1).
When a plurality of EPRs are to address simultaneously the same target EPR
(in other words, when connections are to be set up between terminals from
different origins and terminals connected to the same EPR), they use the
same VPI code. There is no risk of confusion insofar as the connections
are identified in the terminal ends by VCIs which are necessarily
different.
Direct addressing between 2 UCXs
In this case, a link is to be set up between 2 EPRs that do not belong to
the same UCX. The VPI code used by the source EPR is a code that enables
routing to be performed towards the bridge node of the source UCX
connected to the bridge node of the sink UCX. This code is translated into
a local VPI which is that specific to the target EPR. It can be noted
therefore that an EPR must have 28 VPI codes for addressing the EPRs
belonging to another UCX: 14 codes for a route using the 0-ring of the UCX
source and then the 1-ring of the sink UCX. Combinations may also
envisaged comprising the 0-ring then the 1-ring, and the 1-ring followed
by the 0-ring: this would use up as many additional VPI codes.
FIG. 10 shows the two possible connections between the node (C) 10 of the
UCX 103 and the node (P) 102 of the UCX2 104, one of the connections going
via the 0-rings 104, and the other going via the 1-rings 105.
The cells carrying the VPI 107 are transmitted over the 0-ring of the UCX1.
The bridge node (A) is programmed to output them and to translate the code
107 into a code 43. The node (J) of the UCX2 transmits these cells to the
0-ring, the code 43 designates the destination, which in this example is
the node (P). The same technique is used to reach (P) via the 1-rings: the
source code is 123, the translated code being 59. It is recalled that the
codes are taken from the table in Appendix 1, and it can be noted that an
addressing code between 2 UCXs can be deduced easily from the addressing
code within a UCX.
More generally, considering an arbitrary UCX referred to as UCXn, and
starting from this UCX:
1) in order to target a node in the UCXn+1, it is merely necessary to add
the value 64 to the local identifier of the node (in the above example,
43+64=107 and 59+64=123); and
2) in order to target a node in the UCXn+2, it is merely necessary to add
the value 128.
Addressing between 2 Ucxs by transiting via a third UCX
The architecture of the design makes it possible to perform routing between
2 UCXs by going via the third UCX. This is possible by allocating as many
additional VPIs, in this example 28 codes per UCX to be reached after
transiting via an intermediate UCX. The technique uses two translations,
the first being performed in the bridge node of the source UCX, and the
second being performed in the outgoing bridge node of the transit UCX.
FIG. 11 shows the process required for reaching the node (P) 102 of the
UCX2 104 by starting from the node (C) 109 of the UCX1 103 and transiting
via the UCX3 105.
The code 203 switches the cells over the 0-ring of the UCX1 to the node (J)
which directs it towards the UCX3 after translation into 171. Seen from
the UCX3 this code makes it possible to target the node 43 of the adjacent
UCX, in this example the node (P). It is the node (J) which performs the
second translation from 171 to 43.
The code 219 also makes it possible to reach the node (P) by using the
1-rings. It can be noted that it is simple to find the code making transit
possible. 128 is added to 43, then 32 to 171; in the same way, 59+128=187,
187+32=219.
As a general rule:
in order to follow the route UCXn, UCXn+1, UCXn+2 (i.e. UCX1, UCX2, UCX3 or
UCX2, UCX3, UCX1 or UCX3, UCX1, UCX2), it is merely necessary to add 96 to
the address of the target node; and
in order to follow the route UCXn, UCXn+2, UCXn+1 (i.e. UCX1, UCX3, UCX2 or
UCX2, UCX1, UCX3 or UCX3, UCX2, UCX1), it is merely necessary to add 160
to the address of the target node; this case is described above.
For example, if, starting from the UCX1, the node (P) of the UCX2 is to be
reached by transiting via the UCX3:
a) via the 0-rings, the addresses are successively: 203, 171, and 43; and
b) via the 1-rings, the addresses are successively: 219, 187, and 59.
The entire set of routing rules is summarized below:
To go from UCXn to UCXn+1
via the 0-rings: i-a0+64; and
via the 1-rings: i-a1+64.
To go from UCX to UCXn+2
via the 0-rings: 1-a0+128; and
via the 1-rings: i-a1 +128.
Transit: to do UCXn, UCXn+1, UCXn+2
via the 0-rings: i-a0+96; and
via the 1-rings: i-a1+96.
Transit: to do UCXn, UCXn+2, UCXn+1
via the 0-rings: i-a0+160; and
via the 1-rings: i-a1+160.
Setting up point-to-multipoint connections is a requirement of private
local area networks. The apparatus described is capable of satisfying this
requirement, even if the concept of self-routing might appear incompatible
with the notion of broadcasting.
Two approaches may be envisaged:
1) The first consists in managing the multipoint connections without using
the technique described, but rather by relying solely on the conventional
technique of marking by VPI. The broadcast tree is built by marking all of
the translation memories of the MCXs concerned. This unoriginal procedure
suffers from the drawback of requiring a relatively large number of
interchange operations. The MCXs at the center are VPI marked, and those
in the peripheral equipment (EPR) are VCI marked.
2) The second approach consists in performing broadcasting in the center,
and in managing the tree branch by branch in the EPRs. To remain
compatible with the above-described apparatus, it is proposed to allocate
2 VPI codes reserved for broadcasting to each node, one code per ring. Any
cell coming from an EPR and carrying one of the broadcasting codes is
broadcast in the loop over the corresponding ring: after one complete lap,
it is removed by the node via which it entered the loop.
It should be noted that flooding is not total, insofar as the tree uses
only one ring per UCX.
Any broadcasting tree covers all three UCXs, the branches being set up at
the sink EPRs in VCI mode.
FIG. 12 shows the broadcasting technique starting form the node (C) 101
whose VPI code allocated for that purpose is 66 in this example (for
broadcasting via the 0-rings).
The cells carrying the code 66 are transmitted over the 0-ring of the UCX1
and they are broadcast towards each output of the nodes, including the
bridge nodes. The cells are removed via the input node 101 (C), this MCX
having its translation memory programmed to perform this operation.
The bridge nodes 141 (A) and (J) 142 translate the code respectively into
code 77 and into code 64. The cells 72 enter the UCX2 via the node (J),
and the cells 64 enter the UCX3 via the node (A). These codes are
broadcasting codes for the nodes A and J. The above-described process is
therefore repeated in the UCX2 and the UCX3, the 2 input nodes removing
the cells as soon as they have completed an entire lap. However, there is
a difference with respect to the bridge nodes (A) for the UCX2 and (J) for
the UCX3. They are programmed so that the cells are not broadcast at the
outputs, the rule being that a bridge node receiving a broadcasting code
allocated to a second bridge node does not broadcast it via its output. In
this example, (A) of the UCX1 can broadcast 66 because it is allocated to
an EPR node, but (A) cannot broadcast 66 because this code is allocated to
the bridge node (J). This technique prevents cells from travelling
indefinitely over the network.
To sum up:
receiving one of the codes (65, 66, 67, 68, 69, 70, 71, 73, 74, 75, 76, 77,
78, 79) via the 0-ring, a node (A) broadcasts it and translates it into
72; a node (J) broadcasts it and translates it into 64;
receiving one of the codes (81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92,
93, 94, 95) via the 1-ring, a node (A) broadcasts it and translates it
into 88; a node (J) broadcasts it and translates it into 80;
a node (A) receiving 64 via the 0-ring or 80 via the 1-ring removes the
cell;
a node (J) receiving 72 via the 0-ring or 88 via the 1-ring removes the
cell;
a node (A) receiving 72 via the 0-ring or 88 via the 1-ring merely provides
transit; and
a node (J) receiving 64 via the 0-ring or 80 via the 1-ring merely provides
transit.
The above-described principle is applicable to a whole variety of network
architectures provided that their sizes do not exceed a certain limit
fixed by the number of possible codes (in this example, 256 for an 8-bit
VPI field), and taking into account the level of defense that is set on
initializing the network, i.e. the resistance to breakdown, in other words
the number of back-up routes between each pair of access points.
For example:
in a loop having a single ring, only one route is possible;
in a loop having two rings, two routes are possible, and therefore 2 codes
are required per node; and
in a structure made of 2 2-ring loops, four routes are possible, and
therefore 4 codes are required, etc.
Furthermore, the switching elements used in the design described (4.times.4
matrices) make it possible to build architectures having various
topologies to which the mechanism described is also applicable:
a) loop having 1 or 2 rings;
b) conventional matrix stages, e.g. for implementing 16.times.16 switching
units;
c) small switching units cascaded together; and
d) concentration units of various capacities structured in matrix stages.
Moreover, the mechanism is applicable with larger-sized switching equipment
(e.g. 16.times.16 matrices). Its advantages nevertheless become more
difficult to justify insofar as such equipment is relatively bulky (at
least one card), and therefore the memory area is no longer a problem.
Furthermore, the number of messages required for marking is very small.
Architecture may also be provided that implements various types of
equipment, e.g. a center built around 16.times.16 matrices and
distribution networks based on 4.times.4 matrices.
APPENDIX 1: CODE ALLOCATION TABLES FOR THE DESIGN CONSIDERED
The codes identifying the nodes to which the EPRs are connected have been
set arbitrarily as follows:
______________________________________
i.sub.-- a0
i.sub.-- a1
Name of the MTX node
(code as seen
(code as seen
(see FIG. 8) by 0-ring) by 1-ring)
______________________________________
B 32 48
C 33 49
D 34 50
E 35 51
F 36 52
G 37 53
H 38 54
K 39 55
L 40 56
M 41 57
N 42 58
P 43 59
R 44 40
S 45 61
______________________________________
These codes apply for each UCX.
The MCXs referred to as A and J are the access bridges to the other UCXs.
Seen from an arbitrary node of the UCXn by the 0-rings, the nodes of the
UCXn+1 posses the following codes:
______________________________________
VPI target node
i.sub.-- a0 of the target node
______________________________________
96 B 32
97 C 33
98 D 34
99 E 35
100 F 36
101 G 37
102 H 38
103 K 39
104 L 40
105 M 41
106 N 42
107 P 43
108 R 44
109 S 45
______________________________________
The translations from the original codes (on the left) to the target codes
(on the right) are performed in the bridge node of the UCXn which connects
the UCXn to the UCXn+1.
Seen from an arbitrary node of the UCXn by the 1-rings, the nodes of the
UCXn+1 possess the following codes:
______________________________________
VPI target node
i.sub.-- a1 of the target node
______________________________________
112 B 48
113 C 49
114 D 50
115 E 51
116 F 52
117 G 53
118 H 54
119 K 55
120 L 56
121 M 57
122 N 58
123 P 59
124 R 60
125 S 61
______________________________________
Seen from an arbitrary node of the UCXn by the 0-rings, the nodes of the
UCXn+2 posses the following codes:
______________________________________
VPI target node i.sub.-- a0 of the target node
______________________________________
460 B 32
160 + j -- 32 + j
173 S 45
______________________________________
The translations between the original codes (on the left) and the target
codes (on the right) are performed in the bridge node of the UCXn which
connects the UCXn+2.
Seen from an arbitrary node of the UCXn by the 1-rings, the nodes of the
UCXn+2 possess the following codes:
______________________________________
VPI target node i.sub.-- a1 of the target node
______________________________________
176 B 48
176 + j -- 48 + j
189 S 61
______________________________________
In addition to the routing possibilities proposed above, there are those
which make it possible to transit via an intermediate UCX.
Seen from an arbitrary node of the UCXn by the 0-rings, by transiting via
the UCXn+1, the nodes of the UCXn+2 possess the following codes:
______________________________________
i.sub.-- a0 of the
intermediate codes
VPI target node
target node
in the UCXn + 1
______________________________________
128 B 32 96
128 + j -- 32 + j 96 + j
141 S 45 109
______________________________________
The translations between the original codes (on the left) and the
intermediate codes (on the right) are performed in the bridge node of the
UCXn which connects the UCXn to the UCXn+1. The translations between the
intermediate codes and the target codes (in the middle) are performed in
the bridge node of the UCXn+1 which connects the UCXn+1 to the UCXn+2.
Seen from an arbitrary node of the UCXn by the 0-rings, by transiting via
the UCXn+2, the nodes of the UCXn+1 possess the following codes:
______________________________________
i.sub.-- a1 of the
intermediate codes
VPI target node
target node
in the UCXn + 1
______________________________________
144 B 48 112
144 + j -- 48 + j 112 + j
157 S 61 125
______________________________________
Seen from an arbitrary node of the UCXn by the 0-rings, by transiting via
the UCXn+2, the nodes of the UCXn+1 possess the following codes:
______________________________________
i.sub.-- a0 of the
intermediate codes
VPI target node
target node
in the UCXn + 1
______________________________________
192 B 32 160
192 + j -- 32 + j 160 + j
205 S 45 160
______________________________________
Seen from an arbitrary node of the UCXn by the 1-rings, by transiting via
the UCXn+2, the nodes of the UCXn+1 possess the following codes:
______________________________________
i.sub.-- a1 of the
intermediate codes
VPI target node
target node
in the UCXn + 1
______________________________________
208 B 48 176
208 + j -- 48 + j 176 + j
221 S 61 189
______________________________________
In all the above-described cases, when going from one UCX to another UCX,
the cells use the same ring numbers. The principle does not prevent a
change of ring, the constraint being merely additional consumption of
codes. The number of available codes is defined by the number of bits of
the field of the header used for that purpose. In our example, this number
of bits is 8 because the self-routing address is conveyed by the VPI
field. In the above tables, it can be noted that almost all of the 256
codes are used. In addition, changing rings is an option which, like
transiting via an intermediate UCX, is quite justifiable for reasons of
backing up operation (reconfiguration in the event of breakdown).
______________________________________
Codes allocated to broadcasting
Broadcasting
Broadcasting
MCX over 0-ring
over 1-ring
______________________________________
A 64 80
B 65 81
C 66 82
D 67 83
E 68 84
F 69 85
G 70 86
H 71 87
J 72 88
K 73 89
L 74 90
M 75 91
N 76 92
P 77 93
R 78 94
S 79 95
______________________________________
Top